Follow-up question: How was the musket integrated with more traditional weapons in Japan, such as the bow, spear, and sword?

The introduction of firearms to Japan had much the same impact that similar weapons had on most parts of the word. You have two main impacts: first, you can raise more men. Japan used this to great effect in their invasion of Korea in the late 16th century. Being able to raise large quantities of men is useless if they are of inferior quality; the peasants during the Jacquerie outnumbered their opponents but the quality and training of the nobility forces that suppressed them had little trouble in crushing them. When you have firearms, suddenly with a little training huge quantities of effective fighting men can be raised. Second, it weakens the classes of men that traditionally based their power on their fighting prowess and increases the classes that have control over manpower.

In Japan's case, this was the Samurai losing power and the Daiymo gaining power, respectively. The 'dishonour' of the weapon was real, much as the crossbow was considered a 'dishonourable' weapon in Europe, because it weakened those Samurais that relied on their years of training as warriors and their monopoly on martial prowess to maintain their role and station in society, much the same way that knights and minor nobility in Europe during the Medieval period did. In Japan, we saw a roughly analogous change as that which occurred in Europe and that firearms certainly contributed to, which, despite some claims to the contrary, remained a central weapon of the Japanese military throughout the Sengoku period and Tokugawa Shogunate. As in Europe, the old role of the warrior class was not as necessary, but they did not disappear. Instead, their social role remained, backed up by centuries of that social status, rather than the practical element of martial prowess and demonstration.

As for how the weapons were used, the Japanese experimented a lot during the Sengoku period with various methods, much as the Europeans did. Lines of men firing in unison or in serial for a continuous stream of fire were both used. In response to your other question, the way it was used was the same way both the crossbow and firearms in other parts of the world were used initially: to better equip what would otherwise be lightly armed peasant troops. In this sense, we can talk about it adding to the forces of bows and spear among the professional samurai soldiers and replacing the adhoc fighting implements of the peasant armies raised.

Source: A History of Japan, 1615-1867 by George Sansom